Partial Discharge Current Measurements With Small Discharge Gaps Over Polyimide Film

Partial discharge (PD) characteristics obtained with small gaps are very important to determine the PD resistance of polymers in power equipment. Particularly, PD characteristics of polymers used in small-sized power equipment, such as the power module of an electric vehicle instead of a gasoline engine, have become very important. Among polymer insulating materials, polyimide (PI), which is widely used for circuit mounting, is selected as our target insulating film sample. To understand PD mechanisms in depth, we measure nanosecond PD pulse current waveforms instead of the
$\varphi $
-
${q}$
-
${n}$
characteristics, which have been the mainstream of PD studies. Although numerous studies have used the amount of PD charge integrated from the PD pulse current, few studies have measured the high-speed nanosecond PD pulse current itself, which is easily buried in noise. In this study, we measure PD waveforms under an ac peak voltage of 2000 V and more at 50–1000 Hz to clarify the waveforms to understand low-voltage PD characteristics. The electrode system is a simple needle-plane electrode with small discharge gaps of 0, 75, and
$150 ~\mu \text{m}$
. The current direction from the high-voltage electrode to the PI film is determined as a “positive current.” We found that the positive current magnitude is almost twice the negative one, whereas the negative current forms have almost half the duration time of that of the positive ones at almost all applied voltage frequencies. The waveform difference between positive and negative pulse indicates charge origin difference as electrons and ions. This study contributes to the understanding of PD mechanisms.